Proteasome Function and Pathological Proteins in the Pathogenesis of Parkinson’s Disease

Striatal 19S Rpt6 deficit is related to alpha-synuclein accumulation in MPTP-treated mice

Striatal mitochondrial proteins were investigated using proteomics in the 1-methyl 4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. Four proteins, 19S proteasome ATPase Rpt6 (19S Rpt6), Lectin-related nature killer cell receptor LY 49S, Zinc finger A20 domain containing 1, and the sodium channel-associated protein 1 isoform 2, were significantly decreased while alpha-synuclein was increased in MPTP-treated mice. The altered levels of 19S Rpt6 and alpha-synuclein were further verified by Western blot. Experiments using small interfering RNA (siRNA) showed that alpha-synuclein was increased by 50% in cultured striatal neurons when 19S Rpt6 was knocked down. Taken together, our results imply that a deficiency in 19S Rpt6 may be partially related to the MPTP-induced increase in alpha-synuclein in the striatum.

Proteasomal inhibition and apoptosis regulatory changes in human isolated lymphocytes: The synergistic role of dopamine

Abnormal deposition of protein aggregates and increased susceptibility to apoptotic cell death may result from defects in the activity of the ubiquitin-proteasome system (UPS); neurotoxicity related to UPS defects seems to require dopamine to be fully expressed. The aim of this study was to investigate the pro-apoptotic effects caused by proteasomal activity inhibition, as well as the synergistic effect of dopaminergic stimulation in human lymphocytes isolated from healthy volunteers. Cells were incubated 20 h at 37 degrees C, with: (1) lactacystin, (2) increasing concentrations of dopamine or (3) mixture of dopamine and lactacystin. Activities of proteasome 20S and pro-apoptotic caspases-3 and -9 and levels of anti-apoptotic Bcl-2 were measured with fluorimetric or immunochemical assays, while a "DNA diffusion" assay was used to determine the apoptosis. Incubation of lymphocytes with lactacystin, which caused reduction of proteasomal activity, was associated with activation of caspases. A clear, dose-dependent reduction of proteasomal activity was also seen in the presence of increasing doses of dopamine, which was accompanied by a slight dose-dependent increase of caspases activities and Bcl-2 levels. Both effects on proteasome and caspase activities were enhanced when cells were simultaneously exposed to lactacystin and elevated concentrations of dopamine. Apoptosis was detected in all treated samples, but not in controls, without significant differences among the treatment groups; however, the association of dopamine and lactacystin induced a clear reduction in the number of cells being analyzed, pointing to marked cytotoxicity. Our data confirm the potentiation of cytotoxicity related to proteasome inhibition, in the presence of dopaminergic stimulation.

Chaperones and proteases: cellular fold-controlling factors of proteins in neurodegenerative diseases and aging

The formation of toxic protein aggregates is a common denominator to many neurodegenerative diseases and aging. Accumulation of toxic, possibly infectious protein aggregates induces a cascade of events, such as excessive inflammation, the production of reactive oxygen species, apoptosis and neuronal loss. A network of highly conserved molecular chaperones and of chaperone-related proteases controls the fold-quality of proteins in the cell. Most molecular chaperones can passively prevent protein aggregation by binding misfolding intermediates. Some molecular chaperones and chaperone-related proteases, such as the proteasome, can also hydrolyse ATP to forcefully convert stable harmful protein aggregates into harmless natively refoldable, or protease-degradable, polypeptides. Molecular chaperones and chaperone-related proteases thus control the delicate balance between natively folded functional proteins and aggregation-prone misfolded proteins, which may form during the lifetime and lead to cell death. Abundant data now point at the molecular chaperones and the proteases as major clearance mechanisms to remove toxic protein aggregates from cells, delaying the onset and the outcome of protein-misfolding diseases. Therapeutic approaches include treatments and drugs that can specifically induce and sustain a strong chaperone and protease activity in cells and tissues prone to toxic protein aggregations.